The present invention relates to a novel aqueous rust inhibitor. More particularly, the present invention relates to an aqueous rust inhibitor that forms a stable emulsion for exhibiting satisfactory rust inhibiting effects on metals.
A number of aqueous rust inhibitors have so far been reported. They are divided roughly into two groups: inorganic rust inhibitors based on chromates, nitrites or phosphates, and organic rust inhibitors based on various amines, carboxylates (Japanese Patent Publication No. 59309/1982), or acid amides (Japanese Patent Application (OPI) No. 116791/1985 (the term OPI as used herein means an unexamined published Japanese patent application)).
The use of inorganic rust inhibitors is generally not preferred from the viewpoints of safety and environmental protection. For instance, chromates have strong toxicity, nitrites have the potential to produce nitrosamine, and phosphates cause the problem of eutrophication of rivers and lakes. Organic rust inhibitors also are not ideal since their solubility in water is too low to produce as good rust inhibiting effects as inorganic systems. Among the organic rust inhibitors, amines are relatively effective but they present the problem of producing a malodor. Furthermore, the use of organic rust inhibitors cause problems in association with safety and environmental protection although the problems are not as serious as those presented by inorganic systems.
It has been known that hydroxyaryl fatty acids used as one component of the aqueous rust inhibitor of the present invention have rust inhibiting effects. For instance, hydroxyphenylstearic acid is incorporated in ester oils for refrigerators in order to provide corrosion inhibiting effects (Japanese Patent Application (OPI) No. 171799/1986); hydroxyphenylstearic acid or salts thereof with alkaline earth metals are incorporated in hydrocarbons or diesters in order to provide oxidation stability and rust inhibiting effects (U.S. Pat. No. 3,573,333); salts of hydroxyphenylstearic acid and aliphatic amines are incorporated in gasoline fuels to provide corrosion inhibiting effects (U.S. Pat. No. 3,473,902); and salts of hydroxyphenylstearic acid and N,N-disubstituted amines are incorporated in jet fuel to provide corrosion inhibiting effects (U.S. Pat. No. 3,893,825). All of these prior art techniques aim at providing rust or corrosion inhibiting effects by incorporating hydroxyphenylstearic acid or salts thereof with amines or alkaline earth metals in nonaqueous media. In other words, the objective of the prior art techniques is to inhibit rusting in nonaqueous systems and not in aqueous systems as intended by the present invention.
As a mechanism of rust development on metal surfaces may be mentioned the occurrence of electrochemical reaction between coexistent oxygen and water molecules. Successful rust inhibition requires therefore preventing the adsorption of the oxygen or water molecule on metal surfaces. In nonaqueous systems where the oxygen and water molecules are present in much smaller amounts than in aqueous systems, the chance of rust development is inherently so slow that rusting can satisfactorily be prevented with the aid of organic rust inhibitors containing polar groups. On the other hand, effective rust inhibition is extremely difficult to accomplish in aqueous systems where the oxygen and water molecules are present in very large amounts. In order to achieve rust inhibition by suppressing the occurrence of electrochemical reaction on metal surfaces, either inorganic rust inhibitors that passivate the metal surfaces or organic rust inhibitors that form an adsorption layer on the metal surfaces, are employed. However, because of their structure, organic rust inhibitors are only sparingly soluble in water. In addition, the adsorption layer once formed can be disrupted by the water molecules, which are more polar. For these reasons, organic rust inhibitors are essentially less effective than inorganic systems.
In order to compensate for these defects, main organic rust inhibitors to be used in aqueous system generally are based on compounds that have low molecular weights (.ltoreq.300) and large of polarity, and attempts have been made to dissolve these compounds completely in water and to form a stronger adsorption layer on metal surfaces. It has therefore been the general understanding that in the light of their structure, fatty acid soaps having at least 18 carbon atoms are too high in their oiliness to be suitable for use as rust inhibitors in aqueous systems. It has been clear in the prior art that alkaline earth metals such as calcium and amines are effective as soap-forming bases, but it has been entirely unexpected that soaps using alkali metals such as sodium and potassium will also exhibit excellent rust inhibiting effects.
It has been proposed that various compounds made from arylated fatty acids are used in aqueous systems (U.S. Pat. No. 4,597,906). However, these techniques relate to methods of using such compounds as surface active agents, particularly as dispersing agents, wetting agents, emulsifiers or as dyeing auxiliaries and hence differ completely from the present invention whose objective is rust inhibition.
The present inventors previously found that salts of hydroxyaryl fatty acids and amines containing 2-6 carbon atoms would exhibit satisfactory rust inhibiting effects and filed a patent application on an invention based on this finding (Japanese Patent Application No. 281995/1986). However, there still remain problems to be solved since the use of amines causes problems in association with safety and environmental protection although the problems are not so serious as those presented by inorganic rust inhibitors.
In order to be successful, a rust inhibitor must satisfy many requirements but among other things, effectiveness in rust inhibition is most important. However, existing products that have the problems mentioned above are not completely satisfactory for use as industrial rust inhibitors under increasingly diverse and hostile conditions. It has therefore been strongly needed to develop rust inhibitors that exhibit even better performance.